Systemd password agent

[3akev]

Summary

Adds a systemd password agent that answers systemd-ask-password queries via an inotify-watched ask-password directory. Merges the separate polkit panel into a unified auth panel that dispatches between polkit and systemd sources with a priority order (polkit > systemd).

Motivation

See #3792 for the related feature request. In a nut shell, this opens up the possibility of responding to many more password prompts with noctalia's auth panel that was previously reserved for polkit.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

Closes #3792

Testing

Tested manually with pkexec for polkit and systemd-ask-password --no-tty.

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Auth panel with polkit:

Auth panel with systemd-ask-password:

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated user-facing documentation in docs/user/ when this PR changes documented behavior or configuration, or this PR does not require documentation changes.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.